Sulfur and other impurities such as organic nitrogen compounds and olefins are present in a wide range of mostly organic forms in both straight run and refined hydrocarbon streams, including, for example, gasoline, diesel fuel, and kerosene. Sulfur contaminants, while ubiquitous in hydrocarbon products, are suspected of causing adverse environmental effects when converted to sulfur oxides (SOx) upon combustion. SOx emissions are believed to contribute to not only acid rain, but also to reduced efficiency of catalytic converters designed to improve motor vehicle exhaust quality. Furthermore, sulfur compounds are thought to ultimately increase the particulate content of combustion products. Because of these issues, the reduction of the sulfur content in hydrocarbon streams has become a major objective of recent environmental legislation worldwide. The limit for sulfur in the United States, Canada, Japan, the European Community had been at 500 ppm, but recent changes or proposed changes in regulations have called for reducing the maximum limit for diesel sulfur to 5 to 15 ppm, depending upon the applicable regulation.
For the oil refiner, complying with such increasingly stringent specifications has become increasingly difficult as the limits for sulfur and other contaminants have been lowered. In particular, impurities such as thiophenes, organic nitrogen compounds and light olefins share the same boiling point with some desired product streams, such as benzene and toluene and are therefore difficult to remove.
Several prior art disclosures address sulfur contamination in refinery products. U.S. Pat. No. 2,769,760, for example, describes a hydrodesulfurization process with an additional conversion step that does not further reduce the sulfur level but converts some sulfur species to less corrosive forms, allowing the product to meet acidity requirements. Other disclosures are more specifically directed toward essentially complete sulfur removal from hydrocarbons. Particularly, the ability to oxidize sulfur compounds that are resistant to the aforementioned hydrogenation method is recognized in a number of cases. Oxidation has been found to be beneficial because oxidized sulfur compounds have an increased propensity for removal by a number of separation processes that rely on the altered chemical properties such as the solubility, volatility, and reactivity of such compounds. Techniques for the removal of oxidized organic sulfur compounds therefore include extraction, distillation, and adsorption.
In U.S. Pat. No. 3,163,593, organic sulfur compounds contained in petroleum fractions are oxidized by contact with a mixture of H2O2 and a carboxylic acid to produce sulfones, which are then degraded by thermal treatment to volatile sulfur compounds. In U.S. Pat. No. 3,413,307, thiophene and thiophene derivatives are oxidized to sulfones in the presence of a dilute acid. The sulfones are then extracted using a caustic solution. In U.S. Pat. No. 3,341,448, the oxidation and thermal treatment steps are combined with hydrodesulfurization to greatly reduce the hydrocarbon sulfur content. As noted previously, the oxidation and hydrogenation techniques are effective for converting different types of organic sulfur-containing species, thereby leading to a synergistic effect when these methods are combined.
In U.S. Pat. No. 3,505,210, sulfur contaminants in a hydrocarbon fraction are oxidized using hydrogen peroxide or other suitable oxidizing agent to convert bivalent sulfur to sulfones. The hydrocarbon, after having been subjected to oxidation conditions, is then contacted in this case with molten sodium hydroxide to produce a treated product of reduced sulfur content. Another example of a two-step oxidation and extraction method is provided in U.S. Pat. No. 3,551,328, where the extractant is a paraffinic hydrocarbon comprising a 3 to 6 carbon number alkane. Also, EP 0565324 A1 teaches the effectiveness of oxidizing sulfur-containing compounds followed by removal according to a number of possible separations known in the art.
In contrast to the prior art, applicant has determined that organic sulfur contaminants in hydrocarbon feed streams can be removed by a three component catalyst/adsorbent blend. The hydrocarbons purified by contact with this catalyst/adsorbent blend can now be used while the volatile sulfur is easily separable upon regeneration of the catalyst/adsorbent blend.